I think your DM is making a mistake (particularly for a Bard who does not suffer from Arcane Spell Failure from Light Armor to begin with), but if you cannot convince him you can always just work around ASF. It's pretty easy to ensure 0% ASF.
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KRyanApr 20 '13 at 20:51

"A multiclass bard still incurs the normal arcane spell failure chance for arcane spells received from other classes." Other than bard. Does your DM suffer from poor reading comprehension?
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SimanosJan 10 at 14:26

5 Answers
5

Bards can wear light armor without incurring any arcane spell failure chance for their bard spells.

And with heavier armour it's got your back too – it's explicit that you only suffer an Arcane Spell Failure chance with spells that have somatic components:

If the spell lacks a somatic component, however, it can be cast with no chance of arcane spell failure.

However, if this DM has been playing since before 3rd edition, they may have carried forward the "physics" of magic from earlier editions. As the DM this is their prerogative, but if that's the case then they didn't communicate this to you when it was important; if they had, you might have made different equipment choices, or even not chosen to multiclass in the first place.

However, telling the DM they're wrong will likely get you nowhere quickly – if the DM is using a "but my world works this way" house rule, discuss the mismatch of expectations and find out what would be an acceptable solution that would leave you with a character you still want to play while keeping the DM's world consistent. (Consider that the world is their character, and you do want your DM to still want to play it!) If you approach this conversation with self-assurance (but without self-righteousness!) and without putting your DM on the defensive, you'll have a much better chance at a constructive conversation with an outcome that you both find agreeable.

First, make sure your DM understands this is a houserule.

Show him the actual rules regarding this situation in the Player’s Handbook. @DuckTapeal has quoted the relevant section from the System Reference Document, which is official and accurate, but for maximum effectiveness go back to the book.

In addition to that line in the Magic section, the Armor and Weapon Proficiency sections of Bard, Sorcerer, and Wizard all have lines akin to “wearing medium or heavy armor or using a shield incurs a chance of arcane spell failure if the spell in question has a somatic component (most do),” (taken from Bard on the SRD; Sorcerer and Wizard specify all armor).

You are correct that the quoted line about multiclass Bards refers only to spells from other arcane spellcasters. I.e. a Bard/Wizard does not have to worry about light armors’ ASF with his Bard spells, but does for his Wizard spells. Fighter, as a class with no spells, arcane or otherwise, doesn't even begin to come into play there.

So try to make sure the official rule is clear to him. This may not convince him of anything, but it is an important first step.

Second, discuss social contract and expectations.

It sounds like you have already been playing, and he is only now raising this point. Making major houserules like this in the middle of a game is something I have a really strong objection to; if you do as well, that is worth bringing up.

Furthermore, bring up the fact that this is quite weak, is not being done to be a munchkin or anything but literally to take a pretty weak tactic for roleplaying’s sake. Tell him that you expect a certain amount of trust between DM and player; if he really thinks you are trying to mess up his game or something like that, you have more serious issues than multiclassing Bard and Fighter.

Another thing I would bring up is the idea that I expect a good DM to work with players to execute character archetypes in ways that are amenable to the campaign and setting, and to then challenge those characters in ways that are interesting and meaningful to the character. In other words, I expect a certain amount of personal cooperation between DM and players. Forcing you to not play the character you want to play is not being a good DM, in my mind.

Third, show him just how easy it is to ignore this houserule

I’ve listed the most relevant ones for you below. Note that if he’s houseruling how non-Somatic spells work with respect to Arcane Spell Failure, he could easily houserule any or all of these, or may not even have to depending on how many books you have available. That doesn’t make this irrelevant, however. The goal isn’t necessarily to use any of these (though actually I totally do recommend that because restricting yourself to non-Somatic spells is an extremely weak choice), but to demonstrate that what you want to do is an expected and normal part of the system, and to demonstrate how large a houserule this potentially is.

In short, I don’t think Arcane Spell Failure is ever meant to be rolled, you’re merely supposed to have to expend resources eliminating it. If he actually wants you to roll it, he does not understand the system very well. Even a 5% Arcane Spell Failure chance is not viable; sooner or later, and probably sooner rather than later, it means failing to cast a crucial spell when you do not have time to wait a round to cast it again. Under no circumstances should you merely accept it.

The authors of the books seem to have understood this, at least by the end of 3.5’s publishing run. All the ways of mitigating or ignoring ASF have costs associated with them, because it is those costs – not the chance of failure that you should never accept – that are the real balancing measure here. Your DM is not making a small change here, he’s making a huge one. This should reinforce your argument about the social contract. He is overturning a lot of material that expects Arcane Spell Failure to work differently.

Divine Bard Variant from Unearthed Arcana

Unearthed Arcana offers the divine bard variant, which does not have to worry about Arcane Spell Failure at all because his spells aren’t Arcane to begin with. You do suffer from alignment restrictions (pretty strict ones, too), and Divine Bard uses Wisdom for Spellcasting instead of Charisma, but you also get to add a bunch of spells to your list.

Dungeonscape Armored Mage Alternate Class Feature

Dungeonscape allows multiclass Fighter/spellcasters to ignore the Arcane Spell Failure of their armor, regardless of weight, so long as the spell’s level is less than or equal to their Fighter level plus one. This is a pretty bad choice but depending on how many Fighter levels you already have it may be no worse than what you already have.

Complete Arcane’s Battle Caster Feat

Battle Caster from Complete Arcane allows you to ignore Arcane Spell Failure for heavier armors than usual, if you already could for armors of a certain weight. Normally a bad option as it doesn’t give proficiency and has a high feat cost, for a Fighter/spellcaster who already has proficiency and a number of bonus feats, it could work very simply.

Just having 0% Arcane Spell Failure

A mithral breastplate is Light armor, and therefore you, as a Bard, can cast spells in it without paying any heed to Arcane Spell Failure. If you make it a feycraft mithral breastplate and add Thistledown Padding, it also has 0% Arcane Spell Failure anyway.

A +1 twilight feycraft mithral breastplate or a +1 mithral breastplate with Thistledown Padding also has 0% Arcane Spell Failure.

A +1 twilight fey-and-gith-craft mithral full plate with Thistledown Padding has 0% Arcane Spell Failure as well. This is the hardest one to convince a DM of, since having both Feycraft and Githcraft is dubious. Personally, I think this is a really interesting potential plot hook; I wouldn’t just let a player pick one up at a store, but a short side-quest for getting this armor made, convincing a gith armorer and a fey armorer to work together, could be quite fun.

Mithral is Core, twilight is in the Magic Item Compendium or Player’s Handbook II, feycraft and githcraft are in Dungeon Master’s Handbook II, and Thistledown Padding is in Races of the Wild. If any of these are unavailable to you, a single level in Spellsword in Complete Warrior reduces Arcane Spell Failure by 10%, and can replace any two of the last three, or twilight.

The Runesmith from Races of Stone

I have no idea if your character happens to be a Dwarf; with the Charisma penalty, probably not. However, the point of this isn’t to actually be one, it’s to show your DM a class that requires Heavy Armor Proficiency and grants, as one of its major and explicit benefits, the ability to remove Somatic components from all spells so he never has to worry about Arcane Spell Failure. It may help your case.

Fourth, be prepared to either live with it or leave

You may not be successful in your negotiations. In fact, I suspect you won’t be: your DM seems to be extremely narrow-minded and utterly unwilling to show any flexibility on his preconceived preferences, regardless of any argument you might have to make. Furthermore, he seems more than willing to abuse his DM position to enforce his preferences on his players. That is a bad situation.

Your DM is 100% wrong on all points, and furthermore sounds like he has absolutely no interest whatsoever in having any conversation with a player who wants to do anything outside his own extremely-narrow preferences. I'd not play a single session more with him. Not worth it. I believe he has badly gone past the limits of Rule 0.

The rules are quite clear about bards casting spells without somatic components.

The question becomes, what's your next course of action? Since showing him the book and the SRD are insufficient, I doubt pointing him at rpg.stackexchange.com is really going to change his mind. Do you want him to explain exactly what rules changes he is making so that your group can agree upon them? Alternatively, you could tell the DM to stuff it and game with someone else.

A bard can cast bard spells while wearing light armor without incurring the normal arcane spell failure chance. However, like any other arcane spellcaster, a bard wearing medium or heavy armor or using a shield incurs a chance of arcane spell failure if the spell in question has a somatic component (most do).

How you approach this depends on how the GM runs the game. A rules centric GM can certainly be convinced by pointing out the rules (politely!). But for someone that is more story-centric and has put time into the plot and world, that won't work at all. I for instance have a tendency of house-ruling very liberally (but I warn all players of that and the general reasons I will do it at the outset, which matters a lot for the social contract).

For someone more willing to house rule to make the rule you want (or even change one already in place), negotiation can work quite well. In that type of negotiation, first show that it won't unbalance the GM's plot or put you out of balance with the other characters. This is vital, but it could be because it simply won't do that or it could be because you are going to accept some drawback that balances it out. If it will put you out of balance, then its a nonstarter. Then show that you actually want it. Just the fact that you are arguing for it can do that, but showing a willingness to pay for it in character by accepting flaws or spending resources will help.

Often, that will be enough. The game is supposed to be fun. Showing that something will increase your fun without detracting from anyone elses fun will be plenty for many GMs.

But if they are still uncertain, offer something in game that they want. This could be writing a detailed backstory for how you got this thing (here, the lack of spail failure, but it could be almost anything) that contains lots of plot hooks. Agreeing to play along with a particular plan the GM is struggling with is another good one. Helping to shift things towards more tactics or towards more RP opportunities also counts, if that is what the GM is going for. Whatever it is, it should be in game, but as other questions on this site attest, there are often things the GM wants from their players and helping them to get that can help persuade them to give you what you want. This counts whether it is a rule change, a specific item/power you desire, or just a specific plot point.